Description: Thallus foliose, broad-lobed, 2-8(-10) cm in diam., adnate. Lobes irregular, elongate, flat to concave, separate, 5-20 mm wide, 100-200(-300) µm thick, the apices rotund, entire to irregularly cut and isidiate, sometimes upturned, Upper surface dark olivaceous grey to almost black, usually dull, not much wrinkled, homogeneously covered with scattered to dense, granular to subcylindrical, black isidia which are usually darker than the thallus. Upper and lower cortices consisting of a single layer of irregularly isodiametrical cells, internally with loosely interwoven chains of Nostoc and hyphae. Lower surface pale to medium brown, wrinkled, covered in a dense white tomentum of cylindrical hairs (to 0.2 mm long), with long-cylindrical cells. Apothecia very rare, lecanorine, laminal, sessile, 0.5-2.5 mm across, with a brown disc and a smooth to granulose-isidiate thalline margin. Proper exciple euparaplectenchymatous; epithecium brownish; hymenium colourless; paraphyses simple, 1-2 µm thick at mid-level, the apical cells slightly swollen; hypothecium pale yellow. Asci 8-spored, cylindrical-clavate, the apex thick, with a I+ blue apical dome and a I+ blue annulus projecting downwardly. Ascospores submuriform with 3-4 transverse septa and 1 longitudinal septum, hyaline, ellipsoid to subfusiform, 20-25 x 7-10 µm. Photobiont cyanobacterial (Nostoc, the cells in long chains). Spot tests: all negative. Chemistry: without lichen substances.
Note: a cool-temperate to boreal-montane, circumpolar lichen found on bark, rarely on mossy rocks, only locally common, especially in upland areas. The recent record from Tuscany by Pasquinelli & al. (2009), judging from the picture, is most probably wrong and is not accepted here. See also note on L. burnetiae.